1. Field of the Invention
The present invention relates to a polymer having a thiol end group, a process for producing the same, and a block copolymer based on the polymer having a thiol end group. The polymer is obtained by subjecting monomers capable of radical polymerization to radical polymerization in the presence of a thiolcarboxylic acid and then treating the resulting polymer with an alkali or acid.
2. Description of the Prior Art
A thiolcarboxylic acid ester is an important precursor for the synthesis of mercaptans and the introduction of a thiol group, because it is easily decomposed by a base or acid producing a thiol group as a result.
A thiolcarboxylic acid ester is also important for the synthesis of a polymer having thiol groups.
There are proposed some methods for the synthesis of a polymer having thiol groups, as in "Kobunshi Jikken 6" (Experiments in Polymers), pages 367 to 371. According to these methods, vinyl monomers having a thiolcarboxylic acid ester in the side chain undergo radical polymerization and then the resulting polymer is treated with a base or acid so that the thiol group is formed; or a thiolcarboxylic acid is added to the double bond in polyisoprene or polybutadiene to form a thiolcarboxylic acid ester and subsequently the ester is decomposed by a base or acid to form the thiol group.
The thiol groups in a polymer form the secondary structure through the formation of disulfide linkage. This structure plays an important role in activity as observed in proteins and enzymes containing cysteine. Thus it is an extremely interesting subject in biochemistry.
Also in the field of synthetic polymers, many attempts have been made to synthesize a polymer having thiol groups in the side chains. Such a polymer is expected to have an ability to perform oxidation and reduction or an ability to capture heavy metals through the formation of mercaptide. Another effort was directed to the modification of polymers by the polymeric reaction which is induced by the high reactivity of the thiol group. In practice, however, a polymer having thiol groups has not been put to industrial use, because the thiol group is readily oxidized to form a disulfide linkage and the disulfide linkage causes crosslinking, making the polymer insoluble.